Relay type ignition engine tachometer with circuitry for cancelling transient coil currents



March 19, 1968 v. c. WESTBERG 3,374,431

RELAY TYPE IGNITION ENGINE TACHOMETER WITH CIRCUlTRY FOR CANCELLING TRANSIENT COIL CURRENTS 2 Sheets-Sheet 1.

Filed Aug. 27, 1965 N w T C D E R .l. R 4 O Rm 3 m OE T Tm A m VW G N L, w v :9 RW 2 W DWE LL SPARK (POINTS CLOSED) (POINTS OP March 19, 1968 v. c. WESTBERG 3,374,431

RELAY 'TYPE IGNITION ENGINE TACHOMETER WITH CIRCUITRY FOR CANCELLING TRANSIENT COIL CURRENTS Filed Aug. 27, 1965 2 SheetsSheet G u m. mm

N E m mm W R mm [C Y m m. m C a V United States Patent RELAY TYPE IGNITION ENGINE TACHOMETER WITH CIRCUTIRY FOR CANCELLING TRAN- SIENT COIL CURRENTS Vernon C. Westberg, Arlington Heights, Ill. (22 S. State St., Elgin, Ill. 60120) Filed Aug. 27, 1965, Ser. No. 483,065 Claims. (Cl. 324-70) ABSTRACT OF THE DISCLOSURE A relay type ignition tachometer having, in addition to a first relay winding used to commutate a storage capacitor between a meter and a charging source (or a discharging path), an auxiliary winding for suppressing the effect of spark induced current in the first winding upon relay operation.

The present invention relates to a high speed electrical tachometer of the relay type. More particularly, the invention is an improvement upon an electrical relay type tachometer disclosed in my prior Patent No. 3,095,536.

Relay type ignition tachometers of the type described in the above-mentioned patent are operated by contact points of the distributor which, when open, insert the tachometer in series with the ignition coil primary winding and the battery. The relay contacts alternatively connect or commutate a capacitor across a charging and a discharging circuit. The frequency with which the capacitor is charged and discharged is proportional to the rate at which-the engine revolves. By providing a meter responsive to the average current transferred by the capacitor from the charging circuit to the discharging circuit, the frequency of operation of the relay, and therefore the speed of the engine, may be directly indicated.

It is an inherent problem of relay type tachometers that spark induced transients appearing across the primary winding of the ignition coil when the points are opened are also applied across the relay coil. These transients may be sufiiciently large and of a sutficiently long duration to cause temporary release of the tachometer relay known as double stepping resulting in an erroneously high indication on the meter.

It is therefore an object of the present invention to produce more reliable operation of electrical relay type tachometers operated from an ignition coil primary.

More specifically, it is an object of the present invention to improve the accuracy and reliability of electrical tachometers of the type having a relay powered by current through the ignition coil primary by eliminating the deleterious effect of transients appearing across the ignition coil primary upon operation of the relay.

It is another object of the present invention to provide a circuit for a relay type electrical tachometer which not only counteracts currents sent through the relay coil by transient voltages in the ignition coil primary but also provides an improved, constant voltage supply for charging the charge transfer capacitor of the tachometer measuring circuit.

It is still another object of the present invention to provide an electrical tachometer of the relay type which avoids the necessity for separate batteries to supply the meter circuit and which, instead, utilizes electrical energy which is normally wasted or dissipated,

It is a related object of the invention to replace the mercury cells commonly used in electrical tachometers of the relay type with a reference voltage supply which is inexpensive and which provides a sufiiciently high voltage to permit the use of extended scale indicating meters 3,374,431 Patented Mar. 19, 1968 which could not heretofore be used because of their relatively high resistance.

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:

FIG. 1 is a schematic diagram 'of an electrical tachometer constructed in accordance with one embodiment of the present invention, shown connected to a conventional ignition circuit.

FIG. 2a represents a typical voltage wave impressed upon a tachometer relay coil during a spark cycle.

FIG. 2b represents the voltage waves which appear across the relay coil circuit incorporating the subject invention.

FIG, 3 is a partial schematic diagram of a modification of the prior art circuit upon which the present invention as shown in FIG. 4 is an improvement.

FIG. 4 is a schematic diagram of an electrical tachometer constructed in accordance with a preferred embodiment of the present invention, shown connected to a conventional ignition circuit.

While the invention has been described in connection with specific embodiments, it should be understood that I do not intend to be limited to the embodiment shown but intend to cover the alternative and equivalent circuits falling within the spirit and scope of the appended claims.

Referring to FIG. 1, there is shown a typical automotive ignition system including an ignition coil 11 having a primary winding 13 connected at one end through switch 15 and battery 17 to ground. A set of points 19 connect the other end of the ignition coil primary winding to ground, and an ignition point condenser 20 shuts the points 19. The current which normally fiows through primary 13 while the points 19 are closed is interrupted several times during each revolution of the engine by the action of cam 21 which opens contacts 19. The resulting collapse of the magnetic field around primary 13 induces a high voltage across secondary 23 and this high voltage is applied in-succession by distributor 25 to the several spark plugs 24 of the engine, a representative one of which is shown connected between the distributor 25 andground. In a typical engine having eight cylinders with a spark plug in each, four of the plugs are fired during each revolution of the crank shaft. Since each spark is the result of an individual separation of the points 19, the points must be cammed open four times during each engine revolution.

The improved tachometer of the present invention includes a relay 27 having a core 29 and an armature 31 which carries a movable contact 33. The core 29 is wound with a first coil section 35 connected at one end to the ungr'ounded one of the points 13. The other end of coil 35 is connected to ground through a variable resistor 37 shunted by a capacitor 39. The D-C resistance of the ignition primary 13 is relatively low compared to that of the coil 35 so that when the contact points 19 are open, so as to put relay coil 35 in series with ignition coil primary 13 across battery 17, substantially the entire voltage of battery 17 appears across the coil 35, causing its armature to be pulled in.

The movable contact 33 is part of a single pole, double throw switch 34 carried by the relay 27 having a pair of relatively stationary contacts 41 and 43. When the relay is energized, contact 33 engages contact 43. When the relay is de-energized, contact 33 is biased into engagement with contact 41 by spring 42. The switch contacts form part of a circuit for measuring the frequency at which relay 27 is operated. The frequency measuring circuit comprises a charge transfer capacitor 45 connected at ing the capacitor 45 when the relay is energized and a direct current meter 49 for responding to the amount of charge acquired by the charge transfer capacitor 45 when the relay is de-energized. The average current upon rapid cycling becomes a measure of speed. Typically, the capacitor 45 is .47 microfarad, the battery 47 comprises two 1.35-volt mercury cells placed in series, and the meter 49 has a sensitivity such that a current of 835 microamperes through it produces full scale deflection.

The presence of resistor 37 and its shunt connected capacitor 39 in the ground circuit of coil 35 is a feature of the type of electrical tachometer upon which this invention is an improvement and serves to permit such tachometers to operate at high rates of revolution. As described in my earlier issued Patent No. 3,095,536 referred to above, when engine revolutions are high, the time available for establishing and then interrupting a heavy current through the ignition coil primary becomes very short. To insure satisfactory operation of the ignition circuit, the dwell period during which the points 19 are closed to build up a large current through the ignition coil occupies a major part of the spark generating cycle, while the spark period, during which the points 19 are opened so as to cause collapse of the magnetic field and a concomitant spark generating voltage across the ignition coil secondary, is greatly foreshortened. While it is advantageous for the operation of the ignition system to thus cut short the time during which the contact points 19 are opened, such foreshortening makes operation of an electrical tachometer of the relay type increasingly diflicult, since it is during this period, when the contact points are open, that the tachometer relay is energized. In the absence of the resistor 37 and capacitor 39, currents in the coil 35 would cease very shortly after closing of the points 19 and thus the voltage of battery 17 might be applied to the relay coil 37 for an insufiicient time to charge fully the transfer capacitor 45.

Flow of current through the coil 35 is extended into the beginning of the spark period by provision of resistor 37 which, due to a voltage drop across it, charges the shunt connected capacitor 39 during the dwell period. When the ignition points close, current is circulated from the charged capacitor 39 back through the loop 35 in a direction opposite to the previously existing battery current. However, since the relay is not sensitive to the direction of current, this circulating current is effective to maintain the armature pulled in even after the relay has been short circuited by closing of the points 19.

In addition to the problem presented by high speed operation, another problem is inherent in the type of electrical tachometer which is operated from the ignition system of an automobile engine. This problem is the presence of transients induced by a spark across the spark plug and coupled to the primary winding of the ignition coil. FIG. 2a indicates the nature of such induced voltages. When the points 19 open, collapse of the magnetic field associated with the interrupted heavy current in the primary 13 links the numerous turns of the ignition coil secondary 23 and induces a very high voltage across it which is applied to the spark plug causing a dis charge to occur thereacross. This spark discharge causes transient oscillations to be coupled from the coil secondary 23 back to the primary winding 13. As shown in FIG. 2a, these oscillations, which are superimposed upon the positive battery voltage, may be sufficiently large in magnitude to cause the net voltage applied to the relay coil 35 to be negative. During its first few cycles 57-1, the transient wave is of such high frequency that the reactance of the relay coil is sufficient to limit any currents induced by the transient to a magnitude below that which would disturb relay operation. After a few cycles, however, the frequency of the transient voltage is diminished as shown by the waveform at 572. Due to the relatively low reactance presented by the coil 35 to a voltage transient of this lower frequency, the net current which flows through the coil 35 during the negative peak 61 of the transient wave may be negative for a sufliciently long time to cause relay drop out. This phenomenon, also known as double stepping or chattering of the relay contacts, causes erroneous meter readings since the charge transfer capacitor 45 may be commuted between the battery and the meter several times during the spark period thus causing the meter to indicate an erroneously high rate of engine revolutions.

According to an important feature of the present invention tendency toward double stepping caused by sparkinduced transients is reduced by the addition of a second coil portion 51 to the relay, this coil portion being shunted by an asymmetrically conducting device such as diode 53 which may be connected in series with a resistor 55 for limiting current through the loop thus established. Advantageously, the first and second coil portions 35 and 51 may be parts of a single, center-tapped winding. The additional coil 51 operates in conjunction with diode 53 to nullify the effect of negative going spark-induced transients.

More particularly, as seen in FIG. 217, for each transient voltage wave impressed across the relay coil 35 by the induction coil primary 13, an approximately equal but oppositely poled waveform 62 is induced by transformer action across the added coil 51. Thus, during the negative portion of transient 57-2, that current which is attributable to the transient will tend to flow through the coil 35 from the center tap 52 towards the top end of the coil. During this same portion of the cycle, the voltage induced across the coil portion 51 will cause current to circulate through the loop formed by the coil portion 51, resistor 55, and diode 53 in a direction such that the current flow through the coil 51 is from the center tap 52 to the bottom end of that coil. Since the transient induced currents flow in opposite directions in the coil portions 35 and 51, the net magnetomotive force exerted by them is substantially zero, as shown by the waveform 63 in FIG. 2b.

While it is an important part of this invention that the effect of the negative going portions of the transient be nullified, it is not desirable to eliminate the effect of positive portions of the transient since these actually aid the battery 17 and tend to add to the current which it sends through the coil 35. It is for this reason that the diode 53 is employed so as to make the coil 51 inoperative during the positive portions of the transients. Thus, while the current sent through coil 35 by the positive portion of the transient induces by transformer action a voltage across coil 51, this voltage is of such polarity as to be unable to send circulating currents through the coil 51 since it would tend to make the center tap 52 positive relative to the bottom end of the coil. The net magnetic flux due to transient currents during the positive portion of the transient, shown as the waveform 63 in FIG. 2b, is therefore approximately equal to the flux attributable to the current through coil 35.

Whether the charging current into or the discharging current from the charge transfer capacitor 45 is metered is a matter of choice. The tachometer circuit of FIG. 1 is shown connected for measuring the discharge current from the capacitor 45 by connecting the charge transfer capacitor 45 alternately across battery 47 and the meter 49. FIG. 3, upon which the circuit of FIG. 4 is an improvement, shows a circuit connected for measuring the charging current into the capacitor 45. This modification in function is accomplished by connecting the battery 47 in series with the meter 49 and with the charge transfer capacitor 45 so as to supply a metered charge to the capacitor when the movable contact 33 engages the meter contact 41, and by the addition of a discharging resistor 65, connected across the charge transfer capacitor 45 so as to discharge it when the movable contact 33 engages the unmetered stationary contact 43.

FIG. 4 shows a tachometer circuit of the type discussed in connection with FIG. 3 wherein the capacitor charging current is measured and which incorporates, in addition to the feature discussed in connection with FIG. 1, an additional feature whereby the conventional mercury battery 47 is replaced by a storage capacitor shunted across a zener diode. In essence, the transient suppressing loop of FIG. 1 is modified in FIG. 4 by substituting for the resistor 55 a zener diode 67 shunted by a tank capacitor 69. The transient suppressing coil 51 operates in the same manner as it does in the circuit of FIG. 1, with the current induced by undesirable transients circulating through the coil 51, diode 53 and zener diode 67. However, the circulating currents here have the additional effect of charging the storage capacitor 63 to a voltage determined by reverse breakdown voltage of zener diode 67. The diode 53 is connected in opposition to zener diode 57 so that the capacitor 63 is prevented from discharging into the coil 51. Thus the combination of tank capacitor 69 and zener diode 67 represent a source of fixed voltage which replaces the mercury battery 47.

The use of a zener diode and a storage capacitor yields a dual advantage. First, the mercury batteries 47 may be eliminated. Secondly, and perhaps more importantly, the use of the zener diode controlled supply permits in the manner explained in the following paragraph, the employment of a 250 scale meter which would not be practicable with a circuit using mercury batteries.

While a 250 scale meter is obviously desirable in a tachometer because of the increased scale spread which it offers, the use of such a meter has heretofore been difiicult in a circuit powered by mercury batteries because meters of this type have a resistance several times that of meters having a smaller scale spread. The increased resistance would multiply by a factor of several times the time constant of the circuit comprised of the charge transfer capacitor 45 and the meter 47. Due to the very short time available for charging or discharging the capacitor 45 through the meter 49, such an increase is undesirable. The best way of solving the problem introduced by increased meter resistance is to reduce the size of the transfer capacitor 45. While in itself this would reduce the time constant of the capacitor-meter circuit to the desirable amount, reduction of capacitor size would diminish the current delivered through meter 49 below the point necessary to operate the meter. Thus, where meter resistance is increased and time constant is to be kept within desirable limits by a compensating reduction in capacitor size, the rate of current flow through the meter 49 is maintained by increasing the voltage applied to the capacitor 45. When it is recalled that the voltage of a typical mercury battery cell is 1.35 volts and that in circuits designed for use with the smaller degree scale, two mercury cells must be used to delivery sufficient voltage to the charge transfer capacitor 45, it will be realized that to deliver sufiicient voltage through mercury cells to compensate for the increased meter resistance of the 250 meter, four and perhaps six mercury cells would have to be used in series. The need for such a series of mercury cells is eliminated by the zener regulated storage capacitor 69 which can deliver a steady potential of several times that of the previously used pair of mercury cells for powering the charge transfer capacitor 45.

While features of the present invention have been shown in only two particular circuits, the above description of tachometer circuits have suggested several obvious modifications. Thus, while use of the auxiliary, transient nullifying loop 51 has been illustrated for use in a type of circuit wherein the discharge current of a charge transfer capacitor is measured, it will be obvious that the auxiliary coil may be equally well employed in the circuit of FIG. 3 wherein a meter is used for measuring the charging current delivered to the charge transfer capacitor. Similarly, use of the combined transient nullifying and voltage supply circuit made up of the coil 51, diode 53, voltage regulating zener 67 and storage capacitor 69, shown in FIG. 4 as employed in a circuit wherein current delivered to a charge transfer capacitor is being monitored,

the tank capacitor 69 and its associated zener diode 67 are equally well suited to replace the battery 47 in a circuit of the type shown in FIG. 1.

I claim as my invention:

1. In a tachometer for connection to an ignition system having breaker points and a source of current, the combination comprising a relay having a coil arranged for connection to the points and also having first and second fixed contacts and a relatively movable contact, a commutating capacitor in the circuit of the relatively movable contact, a capacitor charging circuit including a source of reference voltage connected to one of the fixed contacts, a capacitor discharging circuit connected to the other of said fixed contacts, a direct current meter connected in one of said circuits to measure the average current therein, an auxiliary coil coupled to the relay coil, and a diode shunted across said auxiliary coil, said diode being poled to conduct when a voltage is induced in said auxiliary coil by transient currents flowing in said relay coil in a direction opposite to the current from said current source.

2. In a tachometer for connection to an ignition systern having breaker points and a source of current, the combination comprising a relay having a coil arranged for connection to the points and having first and second fixed contacts and a relatively movable contact, a commutating capacitor in the circuit of the relatively movable contact, a capacitor charging circuit connected to one of said fixed contacts and a capacitor discharging circuit connected to the other of said fixed contacts, said capacitor charging circuit including an auxiliary coil magnetically coupled to said relay coil, together with a zener diode and a shunting capacitor connected to said auxiliary coil and a direct current connected in series with one of said circuits for measuring the average current therein.

3. In an electrical tachometer for use with an ignition system having a primary circuit including breaker points for grounding the same, said tachometer including a relay having a core, a first coil on said core and means for connecting the same across the breaker points so that the relay is actuated by direct current from the primary circuit when the points are open, and a metering circuit for detecting the frequency of operation of said relay, said reiay being subject to transient current drops caused by spark induced transient voltages in said primary circuit the improvement comprising an auxiliary relay coil on said core connected to the first coil and a diode shunted across said auxiliary coil to circulate compensating currents for overcoming the effect of said transient current drops.

4. For use with an ignition system having a primary circuit loop including an ignition coil primary winding, a source of direct current and breaker points for interrupting said loop, and having a secondary circuit loop including an ignition coil secondary winding magnetically coupled to said primary winding and a spark plug, an improved electrical tachometer comprising:

a relay having a core,

a coil on said core tapped intermediate its ends to form first and second coil sections,

an armature,

first and second fixed contacts and a relatively movable contact,

means for connecting said first coil section to said points so as to send current from said current source through said relay,

a pair of serially connected oppositely poled diodes connected across said second coil section for causing the effect of currents driven through said first coil section by voltages induced across said ignition coil primary by spark discharge across said plug to be selectively cancelled by induced, compensating currents in said second coil section, one of said diodes being a zener diode,

a storage capacitor connected across said zener diode so as to provide a source of reference voltage,

a commutating capacitor connected to said movable contact,

a capacitor charging circuit including said source of reference voltage connected to one of said fixed contacts,

a capacitor discharging circuit connected to the other of said fixed contacts,

and a direct current meter in one of said circuits for responding to the average value of the current therein.

5. For use in an ignition system having a primary circuit including breaker points connected in series between an ignition coil and a battery, an electrical tachometer comprising: a relay having an armature and a coil tapped intermediate its ends to form first and second coil sections, contacts on said relay including a central contact on said armature and two relatively stationary contacts alternatively engaged with said central contact upon energization and release of said relay respectively, means for connecting said first coil section across said points for energizing said relay with direct current when said points are closed, a current source having two terminals and a current meter connected in series across said stationary contacts, a capacitor connected between said movable contact and one terminal of said battery so as to become charged when said central contact engages one of said stationary contacts and so as to become discharged when said central contact engages the other of said stationary contacts, and a diode connected across said second coil section, said diode being poled to conduct when a voltage is induced in said second coil section by spark induced transient currents flowing in said first coil section in a direction opposite to said direct current.

References Cited UNITED STATES PATENTS 3,054,950 9/1962 Cann 32470 3,056,084 9/1962 Parrnater 32470 3,095,536 6/1963 Westberg 324-70 3,134,943 5/1964 Evenson 32470 OTHER REFERENCES Silicon Zener Diode and Rectifier Handbook, Motorola, 1964, pp. 28 and 29.

RUDOLPH V. ROLINEC, Primary Examiner.

M. I. LYNCH, Assistant Examiner. 

